Along with the recent increase in number of the fields where measurement and control of humidity is required, the importance of the humidity sensor has been appreciated.
The humidity sensor which detects humidity by utilizing a change in electric characteristics of the device corresponding to a change in the humidity includes those of an electrolyte type, a metal type, a polymer type and a ceramics type, and various materials have been investigated in each type, however, at moment, humidity sensors of a polymer type and a ceramics type have been practically used. Both types of the sensors utilize their property that the impedance or the electric capacity of the device is changed by absorption and desorption of the water onto the device.
However, the prior art humidity sensors have some problems in their downsizing. When the prior art technique is employed for producing a humidity sensor of a small size, the impedance is raised and the fluctuation of the impedance from one device to another is increased. Besides, the dynamic range of the impedance becomes too big or too small. Accordingly, it has been difficult to produce a small hydrometer of high precision.
Many of the prior art humidity sensors have a slow response, thus it has been impossible to measure the humidity change correctly.
From the view point of the reliability, the prior art humidity sensors are not resistant to a high temperature, high humidity or an organic solvent, and when they are immersed in water, many of them are deteriorated. Furthermore, when they are contacted with an electrolyte solution such as a salt water, even after being dried, the electrolyte remains in the humidity sensing body, and the characteristics are very much changed to make the device unusable. With an organic solvent, as the humidity sensing body is dissolved in it, the characteristics are very much changed and the device becomes unusable.
The present invention is to solve these problems, and its object is to provide a humidity sensor having, in addition to a quick response and high environmental resistance, a low impedance and an appropriate dynamic range of the impedance even after the sensor is downsized.